Accurate and low cost monitoring of transient vibrational, translational or torsional movements of structures having meter scale dimensions is useful for many applications. Of particular interest is monitoring printers, including xerographic, laser, or ink jet printers, for vibrational movements that may range from high frequency acoustic flexure modes (10-20 kHz) to sub-Hz (less than 1 Hz) repetitive or transient mechanical flexure. Vibration measurement can include monitoring for machine breakage, for machine wear (e.g. predictive maintenance monitoring), or even identification of failing components based on characteristic vibration signatures. For certain applications, measuring structure movement or vibration can allow application of tensional, inertial, or acoustic based techniques for vibration suppression or noise cancellation as part of an active vibration control system.
The vibration or transient flexure sensor used in such systems must generally be reliable, accurate, and have low power requirements. In the past, attached strain gauges or interferometric detection systems have been used to detect low frequency or intermittent mechanical flexure of a structural element. Unfortunately, such systems generally suffer from high cost, difficulties in ensuring reliable coupling between the structural element and the detectors, and susceptibility to unreliable measurements due to high frequency vibrations or other interference. In particular, strain gauges can be difficult to calibrate because of thermal and coupling effects, especially when connected to relatively small structural elements that may have their flexural characteristics non-linearly modified by the strain gauge connection.
In contrast, while acoustic microphone based systems are more suited to measure higher frequency, acoustic noise than strain gauges, due to their relatively low sensitivity at low frequencies acoustic microphones still do not provide an optimal solution for vibration measurement over a wide range of frequencies. More recently, micromachined or microfabricated vibration sensors have been tried for vibration detection. For example, microsensors based on etched semiconductor beams, weighted cantilevers, or movable diaphragms constituting a mass and spring system that transforms spring deflection, compression, or extension into a measurable time domain electrical signal have been constructed. The electrical signal is generated or modulated with the use of coupled piezoresistors, piezoelectric materials, or through capacitance changes. Unfortunately, like acoustic microphones, the vibration frequency bandwidth can be limited in such spring type beam or cantilever devices, often being restricted to measurement below the first resonant frequency due to strong non-linearities in spring response. This problem is particularly acute for high vibration frequencies, resulting in reduced overall sensor effectiveness when a large vibration frequency bandwidth is to be measured.
The present invention provides a novel system for measuring vibrational, translational, torsional, flexural or other movements of structural elements using novel partially transparent light beam detectors. A particularly preferred partially transparent light beam detector is a position sensitive detector (PSD) composed of multiple transparent or partially transparent layers to allow passage of a light beam therethrough, with typically more than half of the light (i.e. the measured light intensity) entering the PSD being allowed to exit. Layers may include, but are not limited to, p-i-n detectors having edge mounted electrodes, or appropriately doped amorphous silicon layers. Position sensitive detectors as conventionally defined and defined herein include lateral effect light sensors that produce two electrical signals indicative of the centroid of light beam position. Alone or in combination, partially transparent PSD's in accordance with the present invention can provide one, two, or three dimensional information regarding light beam position.
Alternatively, transparent light beam detectors can include close groupings of transparent photodiodes, or even transparent CCD imaging sensors. In addition, PSD's or other transparent light beam detectors in accordance with the present invention can be modified to have partially reflective layers to redirect a portion of a light beam prior to detection. Combinations of transparent, semi-reflective, and conventional opaque light beam detectors are also contemplated to be within the scope of the present invention, allowing for diverse optical arrangements for measuring light beam position using a single light beam.
Measurement of relative light beam movement is possible over a large range of frequencies, from slow movements having time scales on the order of seconds, to high frequency 10 kHz or greater acoustic measurements. The present invention includes a directed light source for generating a light beam, such as may be provided by a laser beam or highly focused light beam. The light beam is directed to intersect a first partially transparent light beam detector (e.g. a transparent PSD) attached to a first structural element, with the first partially transparent light beam detector allowing at least some of the light beam to exit. Because of the attachment between the light beam detector and the first structural element, detected movement of the light beam with respect to the first partially transparent light beam detector corresponds to movement of the first structural element. To allow for multiple measurement of structure movement, a second light beam detector is attached to a second structural element to intercept the light beam exiting from the first partially transparent light beam detector, with detected movement of the light beam with respect to the second light beam detector corresponding to movement of the second structural element. As will be appreciated, the first and second structural elements can be unitary (e.g. a single flexible beam or panel supporting multiple detectors), coupled by fixed or movable joints (e.g. a linked beam structure, with each beam supporting a detector), or even mechanically isolated from each other.
By tracking one, two, or three dimensional movement of the detected light beam, the flexure or vibration of the structural element over a wide range of frequencies can be determined. Advantageously, the use of partially transparent light beam detectors allows stacking a line of detectors along a beam, sidewall, or other structural element, with all the detectors using the same directed light source to determine movement of the structural element(s). For example, a sidewall of a large printer (having flexural and acoustic characteristics that have been likened to a large metal plate or drum) can be equipped with a number of linearly arranged light beam detectors. A single laser beam can be directed to pass through any line of partially transparent detectors to determine unwanted vibration of the attached sidewall, with appropriate beam bending, splitting, and scanning techniques allowing multiple lines of detectors to be serviced using only a single light source. Using this measured vibration data, vibration or noise canceling techniques can be employed to substantially reduce printer vibration and noise.
Advantageously, the present invention allows vibration monitoring and correction in structural elements maintained in tension, as well as the previously discussed beam, plate or shell type structural elements. For example, in certain xerographic printer systems, an electrode wire or ribbon susceptible to undesired mechanical vibrations is positioned between a donor roll and a latent image to form a powder cloud of toner to develop the latent image. A light beam detector system in accordance with the present invention can be used to monitor vibration modes of the electrode wire, and supply input to a suitable vibration damping control system. One possible control system for canceling mechanical vibration of the electrode wire may be provided by positioning a magnet adjacent to the electrode ribbon, and using low-frequency AC passed through the electrode wire to suppress vibrations through application of electromagnetic forces acting on the wire from the interaction of the AC with the magnetic field.
In addition to meter or sub-meter scale applications such as printers, large scale vibrating or flexing structural elements having linear dimensions measured in meters or even hundreds of meters can be measured in accordance with present invention. For example, tie wires, columns, or beams in buildings or other large structures can be monitored in accordance with the present invention. Suitably scaled or modified flexural or vibration control systems can optionally be used to suppress unwanted structural movements. This would permit monitoring and correction of undesired vibrations or flexures of buildings attributed to large scale movements such as may be encountered in conjunction with wind or earth movements.
Additional functions, objects, advantages, and features of the present invention will become apparent from consideration of the following description and drawings of preferred embodiments.